1. Field of the Invention
The invention relates to compound antifoams comprising alkoxylated partial esters of oligoglycerols, obtainable by condensation of 2 to 20 glycerol molecules, with C.sub.8 to C.sub.22 fatty acids and inorganic or organic solids for defoaming polymer dispersions and aqueous coating systems.
2. Description of the Related Art
In many industrial processes, surface-active substances are employed in a targeted manner in order to obtain particular effects. For instance, aqueous coating materials require a range of auxiliaries, examples including emulsifiers to emulsify the water-insoluble binders, or additives to improve substrate wetting and pigment dispersion. An undesirable side effect of these surface-active substances, however, is that they stabilize, in the form of foam, the air introduced in the course of preparation or application.
The use of silicone oils, especially dimethylpolysiloxanes of low to medium viscosity, for defoaming aqueous solutions or dispersions is known and is described, for example, in the book by W. Noll "Chemie und Technologie der Silicone" [Chemistry and Technology of the Silicones]. To improve the defoaming action of organosilicon antifoams and/or those based on mineral oil it is common to add highly disperse inorganic or organic substances, especially silicas produced generally by the pyrogenic method (DE-C-10 67 003, DE-A-19 14 684). The use of polyoxyalkylene-polysiloxane copolymers as antifoams is likewise known (U.S. Pat. No. 3,763,021).
These prior art formulations comprising silicone oils or polyoxyalkylene-polysiloxane copolymers are suitable to a greater or lesser extent for preventing the formation of foam in polymer dispersions or aqueous coating systems or for destroying existing foam. It has been found, however, that aqueous coating systems to which formulations of this kind have been added have disadvantages which are attributable to the addition of these antifoams.
It has been found in particular that high-gloss emulsion paints to which polysiloxanes, polyoxyalkylene-polysiloxane copolymers or formulations based on mineral oil have been added to remove foam, exhibit a variety of wetting defects, and also reduce gloss, when they are applied to surfaces. These defects are manifested in nonuniform wetting of the substrate, and lead to the formation of coatings of irregular thickness and, in extreme cases, to coatings containing defect sites of varying extent.
Moreover, the use of silicone-based products in aqueous coating systems often provokes problems of interlayer adhesion; when used in dipping tanks, it is difficult to avoid the occurrence of craters over a prolonged period; and, in application processes that are followed by catalytic post-combustion, the service life of the catalysts is reduced.
In addition, the use of products based on mineral oil is in many cases inopportune. In addition to the physiological and ecological risks, these carrier oils reduce--in aqueous media, for example--the degree of gloss of emulsion paints or lead, in flexographic printing inks, to unwanted swelling of the flexographic plates.
DE-A-36 36 086 describes fatty acid esters of polyglycerol polyglycol ethers that are obtained by conventional reaction of polyglycerols with ethylene oxide and propylene oxide and by esterifying the resulting alkylene oxide adduct with fatty acids, using from 2 to 15 mol of ethylene oxide, from 10 to 60 mol of propylene oxide and from 1 to 3 mol of fatty acid per mole of hydroxyl groups present in the polyglycerol. In aqueous coating systems, however, the foam prevention properties of these products are inadequate because it is not possible to obtain the necessary long-term action in the paint.